1. Field of the Invention
This invention relates to an epitaxial wafer and methods of manufacturing it. More precisely, this invention relates to an epitaxial wafer used in light emitting elements such as light emitting diodes, and methods of manufacturing it.
2. The Prior Art
Light emitting elements, such as light emitting diodes, are obtained by laminating semiconductor layers on top of a semiconductor single crystal substrate (hereafter referred to as "substrate wafer") to fabricate a semiconductor substrate with a pn junction and then making it into an element(s). For example, GaAlAs light emitting elements are manufactured by using an epitaxial wafer obtained by forming, by means of epitaxial growth, a p-type GaAlAs clad layer, a p-type GaAlAs active layer and a n-type GaAlAs clad layer (hereafter, these are generically referred to as "GaAlAs layers") one after another on a p-type GaAs substrate.
The following process, for example, has been conventionally employed to manufacture the GaAlAs epitaxial wafers as described above with the liquid phase epitaxial method. As shown in FIG. 7, wafer holders 61 on which p-type GaAs substrate wafers 70 are stuck are placed in a parallel fashion at prescribed intervals in the carbon container 60, then this container 60 is filled with the Ga solution 62 in which Al and GaAs crystals are dissolved, and the temperature is lowered at a prescribed rate to obtain epitaxial growth layers of GaAlAs on the p-type GaAs substrate wafers 70.
The GaAs substrate wafer portion of the epitaxial wafer thus obtained is eventually either partially removed to have a prescribed thickness or completely removed, by means of etching and such. That is, the epitaxial wafer described above needs enough strength to survive various treatments during the epitaxial wafer manufacturing process, and the GaAs substrate wafer provides that strength. Therefore, at the final stage after the treatments, the presence of the GaAs substrate wafer is not necessarily required.
For example, when manufacturing light emitting elements which are not required to have a very high luminance, relatively thin, 10-50 micrometers, GaAlAs layers are formed on the GaAs substrate wafer, and eventually a prescribed thickness from the GaAs substrate side is lapped and/or etched away to adjust the total thickness. When manufacturing light emitting elements which are required to have a high luminance, thick GaAlAs layers, about 200 micrometers for example, are formed on the GaAs substrate wafer, and eventually the GaAs substrate wafer is completely removed by means of lapping and/or etching.
However, in conventional methods such as the one described above, a substantial portion of the semiconductor single crystal which was the material of the substrate wafer was wasted, causing higher costs. That is, a substrate wafer of a thickness of approximately 300 micrometers was usually used to guarantee the substrate strength during the epitaxial wafer manufacturing process. However, since the wafer strength was not needed as much after the epitaxial layers are formed on the substrate wafer, the substrate wafer was eventually lapped and/or etched down to a prescribed thickness or completely removed by means of lapping and/or etching. Therefore, that amount of semiconductor single crystal ingot (hereafter referred to as "crystal") was wasted.
Also, there was a productivity problem in that the number of wafer holders which can be placed in said container was limited and therefore the number of substrate wafers to be treated in one epitaxial growth process was limited in the epitaxial growth process described above, wherein the epitaxial growth process was conducted in a container accommodating a plurality of wafer holders onto which the substrate wafers were stuck. It was also time consuming to stick the substrate wafers onto the wafer holders.